This invention relates to television signal processing, in particular to the reduction of distortion of video images transmitted in line sequential chroma format.
As a result of recent advances in digital electronics that permit a greater variety of video image modifications, the television industry has experienced a need for an increased number of editing passes of a television signal to implement such modifications. Since present signals, based on the method of frequency multiplexing adopted by the National Television System Committee (NTSC) in 1953, suffer noticeable degradation after a few video tape transfers, there is a need for alternative coding methods less subject to distortion by such processing. While a higher quality signal less subject to such degradation could be provided with increased bandwidth, there is, at the same time, a need to maintain the signal within the existing bandwidth in order to continue to utilize existing transmission equipment, and to minimize technical complexity and expense. It is therefore desirable to code the signal so as to make maximum use of the available bandwidth with minimal visually perceptible degradation in the video image.
One approach to this problem is to use a line sequential chroma format. In this format, the video signal, which comprises a luminance (intensity) component (Y), a first chrominance or "chroma" (color) component (C.sub.1), and a second chroma component (C.sub.2), is transmitted so that for each horizontal line of the television raster, only one of the two chroma components is transmitted with the associated luminance component, each chroma component being transmitted alternately with respective segments of the luminance component. Thus, for example, a first luminance segment Y.sub.1, corresponding to a first horizontal line, would be accompanied by a segment of a first chroma component C.sub.11 ; a second luminance segment Y.sub.2, corresponding to a second horizontal line, would be accompanied by a segment of a second chroma component C.sub.22 ; a third luminance segment Y.sub.3, corresponding to a third horizontal line, would be accompanied by a segment of the first chroma component C.sub.13 ; a fourth luminance segment signal Y.sub.4 would be accompanied by a segment of the second chroma component C.sub.24 ; and so on. Thus, one of the two chroma components for every other line is deleted.
In order to have a complete picture displayed at the receiving end, the deletion of every other segment of each chroma component must be accommodated. The necessity for, and manner of, accommodation is dependent upon the manner in which the line sequential signal is transmitted, which may be by frequency multiplexing or time division multiplexing.
An example of a frequency multiplexed line sequential color transmission system is the SECAM (SEquential Couleurs A Memoire) system used in France, L. E. Weaver, The SECAM Color Television System (Tektronix, Inc., 1982). Because the chroma sequence in SECAM spans two frames the human eye will average the consecutive chroma signals to get the right color. However, such a system introduces perceptible and visually undesirable frame flicker.
Another approach is exemplified by a "time compressed color component" (TC3) system employing time division multiplexing. In this system the two chroma components are compressed in time and transmitted alternately before a corresponding segment of the luminance component. At the receiving end the missing chroma segments are reconstructed by simply inserting the corresponding chroma segment for the previous horizontal line. A system of this type has been described in Charles W. Rhodes, "Time Division Multiplex of Time Compressed Chrominance for a Compatible High Definition Television System," Proceedings of the International Conference on Consumer Electronics, Chicago, Ill., June 11, 1982. A similar system known as the multiplexed analogue component method has been described in K. Lucas and M. A. Windram, Direct Television Broadcasts by Satellite Desirability of a New Transmission Standard, Independent Broadcasting Authority Report 116/81 (IBA, Crawley Court, Winchester, Hants, SO21 2QA, United Kingdom).
One problem resulting from the use of line sequential chroma format is that the chroma signal contains spatial frequency components at frequencies greater than one-half the vertical spatial sampling rate. This produces aliasing which materializes as visually perceptible, and undesirable, distortion in the video picture. It has been recognized that this problem can be overcome by prefiltering the video signal to eliminate frequencies greater than one-half the line sequential chroma sampling rate. Rhodes paper, supra. Nevertheless reconstruction of the missing chroma segments at the receiver results in the generation of phantom high frequency components which produce undesirable visual artifacts. There is therefore a need to reconstruct the chroma segments without the artifacts produced in a time division multiplexed line sequential system and without the undesirable flicker produced in a frequency multiplexed system.
Another problem raised by the use of a line sequential system is that such a system tends to increase the complexity of the transmission and reception apparatus. It would be desirable to implement such a system with minimal complexity at the receiver, which is typically a consumer component, particularly for use in direct satellite broadcast systems which would employ converters connected to existing television receivers.
A further approach to the reduction of bandwidth in a television system is the use of two dimensional sampling. For example, the use of horizontal offset sampling in high definition television has been described in Broder Wendland, "High Definition Television Studies on Compatible Basis With Present Standards," Television Technology in the 80's (Society of Motion Pictures and Television Engineers, 1981).
It has been found that in horizontal sampling systems increased horizontal resolution can be achieved by using offset sampling, that is, where the samples in each consecutive horizontal line are offset horizontally from their predecessor. While it would be desirable to use horizontal offset sampling in connection with a line sequential chroma format, sampling in the horizontal dimension produces the same problems of aliasing and reconstruction as is produced by sampling in the vertical dimension.